#include<bits/stdc++.h>
#define XH(i,a,b) for(int i=a;i<b;++i)
#define XXH(i,a,b) for(int i=a-1;i>=b;--i)
#define LL long long
using namespace std;
int tmpINT; string tmpSTRING; char tmpCHAR; double tmpDOUBLE; LL tmpLONGLONG;

int V,T,S,D;

int main(){
	scanf("%d%d%d%d", &V,&T,&S,&D);
	double time = D*1.0/V;
	if(time>=(double)T&&time<=(double)S){
		printf("No\n");
	}else{
		printf("Yes\n");
	}
	
	return 0;
}